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2007 OR10 has a moon

An article published in “The Astrophysical Journal Letters” describes the discovery of a moon of the 2007 OR10 transnettunian object. It’s most likely a dwarf planet which is still little known because right now it’s distant from the Sun about 87 times the Earth. A team of astronomers led by Csaba Kiss of the Konkoly Observatory in Budapest analyzed images of the Hubble Space Telescope’s archive finding two images of 2007 OR10’s moon.

2007 OR10, or more precisely (225088) 2007 OR10, is a scattered disc object, a category of trans-Neptunian objects that is part of a region rich of icy planetoids. The considerable distance made it difficult to make observations precise enough to be able to determine its characteristics and classify it correctly. Almost exactly a year ago a research published in “The Astronomical Journal” based on observations made with the Kepler and Herschel space telescopes provided a new estimate of its diameter in 1.535 kilometers (955 miles).

Based on these estimates, 2007 OR10 would be the third transnettunian object in size after Pluto and Eris and should therefore be a dwarf planet. The discovery of a moon could help establish its classification for good if it was possible to determine its orbit so that it would help estimate the mass of 2007 OR10, a further important piece of data to determine if it’s a dwarf planet according to the definition of the IAU.

Most dwarf planets have at least one moon and 2007 OR10 has an estimated rotation period of 45 hours, whereas Kuiper’s objects usually have periods of less than 24 hours. This strangeness convinced Csaba Kiss and his team to examine the 2007 OR10 images taken using the Hubble Space Telescope on occasions when it was used to observe it.

As can be the case for research conducted even years away, this archive turned out to be a small treasure in two images taken on November 6, 2009 and September 18, 2010. The traces of a small object near 2007 OR10 were really weak, so much that only a specific search identified them.

If 2007 OR10’s moon had the same albedo as the dwarf planet, its diameter would be 237 kilometers (about 147 miles). Thanks to the infrared observations conducted with the Herschel Space Telescope, the astronomers obtained some other information that allowed them to offer an estimate of this moon’s size, though rough, between 240 and 400 kilometers (between 150 and 250 miles) in diameter.

The recent discoveries of moons orbiting dwarf planets suggest that in the early stages of the solar system’s history collisions were common and in some cases they generated those moons. However, only collisions at the right speeds had that effect: those at too high speeds would have made the debris escape the dwarf planet’s gravity, those at too low speeds would have only generated a crater.

For these reasons, the discovery of another moon around a dwarf planet provides us with more information about the history of an area still little known of the solar system. The latest generations of instruments are allowing us to better understand how much the solar system is really big and this is just the beginning because the hypothesis that there’s even a planet and the theories about the Oort cloud show that there’s still a lot to discover.